Retrogradation of Waxy Rice Starch Gel in the Vicinity of the Glass Transition Temperature

The retrogradation rate of waxy rice starch gel was investigated during storage at temperatures in the vicinity of the glass transition temperature of a maximally concentrated system ( ), as it was hypothesized that such temperatures might cause different effects on retrogradation. The value of fully gelatinized waxy rice starch gel with 50% water content and the enthalpy of melting retrograded amylopectin in the gels were investigated using differential scanning calorimetry. Starch gels were frozen to ？30°C and stored at 4, 0, ？3, ？5, and ？8°C for 5 days. The results indicated that the value of gelatinized starch gel annealed at ？7°C for 15？min was ？3.5°C. Waxy rice starch gels retrograded significantly when stored at 4°C with a decrease in the enthalpy of melting retrograded starch in samples stored for 5 days at ？3, ？5, and ？8°C, respectively, perhaps due to the more rigid glass matrix and less molecular mobility facilitating starch chain recrystallization at temperatures below . This suggests that retardation of retrogradation of waxy rice starch gel can be achieved at temperature below . 1. Introduction The retrogradation of starch has been defined as a process which occurs when the molecules comprising gelatinized starch begin to reassociate in an ordered structure [1]. Retrogradation can lead to an obvious increase in the firmness of stored baked goods [2] and frozen cooked rice [3], making them unattractive to consumers. However, in some products, retrogradation can provide a desirable quality such as in the manufacture of rice stick noodles [4], resistant starch type 3 [5] croutons, and bread crumb [6]. For this reason, numerous studies have been performed to examine the factors affecting the retrogradation of starch. Water content and storage temperatures play key roles in the extent of retrogradation. The maximum extent of retrogradation of most starches is attained in starch gels containing 50%–60% solids [7–10]. It was also found that starch gels retrograde faster when stored at 4？6°C [11, 12]. Glass transition is a second-order phase transition that occurs over the temperature range at which amorphous solid materials (glassy materials) are transformed to a metastable leathery state [13]. A special glass transition temperature, denoted as , has been defined as the glass transition of a maximally freeze-concentrated system [14]. plays a key role in the quality and storage stability of frozen products because the rate of deteriorative changes in frozen food is closely related to [13, 15]. Below , where the food matrix is in a glassy state,